Electro-optic silicon modulator

ABSTRACT

The present invention provides an electro-optic modulator and an optical communication system in which a wider signal electrode may be used without affecting the characteristic impedance of the device or the efficiency of the optical modulation. In embodiments of the invention, asymmetric coplanar electrodes are provided such that the gap between the signal electrode and one reference electrode may be optimized for the optical waveguide and the semiconductor section surrounding it, and the gap between the signal electrode and the other reference electrode may be optimized for a particular characteristic impedance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to the patent application of GreatBritain No. GB 1108481.1 “Electrodes for high speed modulators” filed onMay 20, 2011.

FIELD OF THE INVENTION

The present invention relates to the field of electro-opticalmodulation, and particularly to electro-optic modulators with novelelectrodes and modern high data rate optical communication systemscomprising the same.

BACKGROUND ART

Silicon microphotonics has generated an increasing interest in recentyears. Integrating optics and electronics on the same chip would allowenhancement of integrated circuit (IC) performance. Furthermore,telecommunications could benefit from the development of low costsolutions for high-speed optical links. The realization of activephotonic devices, in particular high speed optical modulators integratedin silicon-on-insulator (SOI) waveguides, is essential for thedevelopment of silicon microphotonics/nanophotonics.

Although silicon does not in normal circumstances exhibit a linearelectro-optic (Pockels) effect, other mechanisms are available formodulation, including thermo-optic and plasma dispersion effects. Asidefrom these, further interesting methods have been reported which includeusing strain to introduce a Pockels effect, forming SiGe/Ge quantumwells to take advantage of the quantum-confined stark effect, andbonding III-V materials to make use of their stronger electro-opticproperties. The disadvantage of these approaches is the complex ornon-CMOS compatible fabrication processes involved. The thermo-opticeffect in silicon is relatively, very slow and therefore has no real usefor high speed applications. The plasma dispersion effect on the otherhand is much more promising with most of the recent successfulhigh-speed silicon modulators being based upon this effect, whilst usingcarrier injection, depletion or accumulation to cause the requiredchanges in free-carrier concentration.

The plasma dispersion effect uses changes in the free-carrierconcentration to cause modulation of the light passing through thedevice. The free-carrier concentration may be changed by injectingcarriers into the device, depleting carriers from a region of the deviceor by causing an accumulation of charge carriers in a region of thedevice. Carrier injection is typically carried out in a PIN diodestructure with the optical waveguide passing though the intrinsicregion. When the diode is forward biased, carriers pass into theintrinsic region causing a change in refractive index. Carrier depletioncan be based upon a PN junction diode in the waveguide. Reverse biasingthe diode causes carriers to be swept out of part or all of thewaveguide region, again resulting in a change in refractive index.Carrier accumulation involves the use of an insulating layer between Pand N diode regions that will, when biased, cause an accumulation offree carriers on the edges of the layer, much like a capacitor. Carrierdepletion and accumulation, unlike carrier injection, are not limited bythe relatively long minority carrier lifetime in silicon andconsequently the fastest reported devices have utilised thesemechanisms.

Electrodes with high performance are required to drive the electricalinput signal along the length of the optical modulator. The performanceof the electrodes can dictate to some extent the overall performance ofthe optical modulator and therefore careful consideration of theirdesign is required. Optical modulators can range in length from hundredsof micrometres to several millimetres and even centimetres.

There are three main considerations when designing the electrodes of anoptical modulator.

1. The loss of the electrode (i.e. the attenuation of the electricalsignal along the length of the modulator) should not be too high, anddependence of the loss on the frequency of the electrical signal shouldalso be limited.

2. The electrode (including the effect of the optical modulator) shouldhave characteristic impedance matched to that of the driving system andtermination.

3. The velocity of the electrical signal along the electrode shouldmatch the velocity of the light propagating through the modulator.

The first consideration is important in ensuring both efficient use ofthe drive signal and a high modulation bandwidth. If the drive signalattenuates as it passes along the electrode then the phase shiftproduced will be less than if the full voltage is applied along theentire length of the device. Furthermore, frequency-dependent loss meansthat higher frequency components of the drive signal will be attenuatedat a faster rate than low frequency components. This can limit thebandwidth since the modulation depth achieved at low frequencies will belarger than at high frequencies.

The second consideration relates to the fact that maximum transfer ofthe drive signal to the electrode is achieved when the characteristicimpedances of the driving system and the electrode match at theboundaries between those two components. Similarly, by matching thecharacteristic impedances of the electrode and the terminatingcomponents, resonances in the system as a result of reflected signalscan be reduced, which could otherwise degrade the performance of thedevice.

The final consideration is important as it can again set a modulationbandwidth limitation. If there is a mismatch between the velocities ofthe electrical signal and the light propagating in the waveguide, themodulation may be smeared over a longer period of the light. As thefrequency of the drive signal is increased the edges of each bit, whichbecome broadened due to the modulation smearing, start to overlap andinterfere with each other.

In terms of the semiconductor part of the device (which also makes upthe waveguide), the electrical contacts to the device should be as closeto each other as possible in order to have a high modulation bandwidth.On the other hand, if the contacts are too close to the waveguide suchthat they interact with the propagating light, undesirable optical losscan be caused. There is therefore an optimum separation of the twoelectrical contacts set by the semiconductor element and this dimensionshould be taken into account when designing the layout of theelectrodes.

Coplanar waveguide electrodes are typically used to bias the waveguideas they do not require a ground plane on the back-side of the substrate.One coplanar waveguide structure 10 is shown in plan view in FIG. 1(prior art). Only the electrodes are illustrated for clarity. A signalelectrode (also known as a signal track) 12 is surrounded on either sideby two ground planes 14, 16. The semiconductor section of the device(comprising the optical waveguide) is not illustrated, but runs parallelto and between the signal track and one of the ground planes. The signalrunning down the signal track 12 provides a biasing potential whichaffects the dispersion characteristics of the waveguide.

It will be appreciated that the above problems are peculiar tomodulators formed of silicon which, as described above, does not exhibita strong or linear electro-optic effect so other mechanisms (such as theplasma dispersion effect) are used to effect modulation.

A wide signal track results in lower electrode loss (and lowerfrequency-dependent loss). However, in order to maintain impedancematching with the drive circuitry, the gap width between the signaltrack and the surrounding ground planes should be increasedcorrespondingly. This is because the characteristic impedance of thecoplanar waveguide electrode is dependent on the ratio of the signaltrack width and the gap widths.

This leaves three choices:

1. Increase the signal track and gaps correspondingly, so the impedanceremains the same. However, this has a detrimental effect on thebandwidth of the semiconductor section as the gap between the contactsis increased.

2. Use a relatively thin signal track but choose an optimal gap widthfor the semiconductor section. This has a detrimental effect on thebandwidth of the electrode.

3. Increase the signal track width without increasing the gaps betweenthe signal track and the ground planes. However, in this case thecharacteristic impedance of the modulator will no longer match that ofthe drive circuitry and termination components, resulting in reflectionsof the drive signal.

None of these solutions is ideal. And there is a need for find anoptimal solution to overcome the existing problems.

SUMMARY OF THE INVENTION

The present invention addresses the use of an asymmetric coplanarwaveguide in optical modulators, having non-equal gap widths on eitherside of the signal track. This allows for a wide signal track withoptimal gap on one side (in terms of the performance of thesemiconductor section) and a gap on the other side used to tune theimpedance of the device as required.

In one aspect, the present invention provides an electro-optic modulatorcomprising: an optical waveguide integrated in a layer of silicon; andbiasing circuitry for applying an electric potential across thewaveguide. The biasing circuitry comprises a signal electrode coupled toone side of the waveguide, for application of an electrical signal; aprimary reference electrode coupled to the other side of the waveguide,for application of a reference signal; and a secondary referenceelectrode, also for application of said reference signal, located suchthat the signal electrode lies in a coplanar arrangement between thereference electrode and the secondary reference electrode. A first gapbetween the signal electrode and the primary reference electrode and asecond gap between the signal electrode and the secondary referenceelectrode are not equal.

By permitting an asymmetric electrode system, embodiments of the presentinvention allow the first gap to be tuned or optimized for the opticalwaveguide, while the second gap can be tuned or optimized to set thecharacteristic impedance of the modulator. This in turn allows a widersignal electrode to be used, increasing the bandwidth of the modulator.

In an embodiment of the present invention therefore, the first gap has aminimized width such that the signal electrode and/or the primaryreference electrode does not interact with light propagating in thewaveguide. In a further embodiment, the second gap has a width such thata characteristic impedance of the biasing circuitry takes a desiredvalue.

The terms “gap” and “width” will be understood within the context of asignal electrode which is elongate and generally runs parallel to theprimary and secondary reference electrodes, i.e. such that the gaps havesubstantially constant widths.

In an embodiment of the present invention, the signal electrode has awidth in the range from 5 to 20 μm. In further embodiments, at least thesignal electrode and the primary reference electrode have a thickness inthe range from 1 to 5 μm.

Where the optical waveguide is formed in a first layer together withelectronic components, it may not be possible to increase the electrodethickness to these values. In an embodiment, therefore, the biasingcircuitry is located on a second, higher layer with greater thicknessthan said first layer, and contact down to the optical waveguide. Theterms “first layer” and “second layer” as used herein are not intendedto refer to any metal layer in particular, but rather to any twodifferent metal layers.

According to a second aspect of the present invention, there is providedan optical communication system, comprising: an electro-optic modulatoras set out above; a source of photons, coupled to the waveguide; anddriving circuitry coupled to the biasing circuitry, for generating theelectrical signal.

In an embodiment, the second gap is set to have a width such that acharacteristic impedance of the biasing circuitry is equal to acharacteristic impedance of the driving circuitry at a boundary betweenthe two. In this way, reflections of the electrical signal are reducedor minimized.

In a yet further embodiment, the electrical signal alternates at afrequency f, resulting in a skin depth δ (according to the equationgiven below). In this case, at least the signal electrode and theprimary reference electrode have a thickness in the range from 2δ to 5δsuch that the AC current of the electrical signal is spread through theconductor as much as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures in which;

FIG. 1 (Prior Art) is a plan view of a conventional coplanar waveguidestructure;

FIG. 2 shows an optical modulator according to embodiments of thepresent invention;

FIG. 3 shows a plan view of a coplanar waveguide structure according toembodiments of the present invention;

FIG. 4 is a graph illustrating the bandwidth of electrodes withdifferent thicknesses; and

FIG. 5 shows an optical communication system according to embodiments ofthe present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 shows in cross-section the optical part of a modulator 100according to embodiments of the present invention. A layer oflight-carrying material (silicon) is formed on an insulating substrate120 (such as SiO₂). The insulating layer 120 is frequently referred toin the art as a ‘buried oxide’ layer. However, alternative materialswill be apparent to those skilled in the art, such as (but not limitedto) silicon on sapphire (where the entire sapphire substrate isinsulating), germanium on silicon, and silicon-germanium on insulator.

The light-carrying material has a thicker portion known as the “rib”122, which acts as a waveguide and along which photons propagate whenthe modulator 100 is in use. Insulating cladding (e.g. SiO₂) 124 isprovided to protect the waveguide from damage and to reduce opticallosses. The device 100 would also work with an air (or other) cladding,however (i.e. without the insulating layer 124). A signal electrode 112and one reference electrode 114 are coupled to the light-carrying layeron either side of the waveguide 122, such that an electric potentialbetween the two effectively biases the light-carrying material and thewaveguide 122.

The light-carrying material is divided into differently doped regionssuch that a pn junction is formed somewhere between the two electrodes112, 114. In the illustrated example the pn junction is formed in thewaveguide 122; however, it may also be formed to the side of thewaveguide. Regions of higher doping concentration are formed at theconnections to the respective electrodes.

The p- and n-type regions are typically doped at a concentration ofbetween about 10¹⁶ and 10¹⁸ cm⁻³; and the p+ and n+ regions doped at aconcentration of between about 10¹⁸ and 10²⁰ cm⁻³, although differentconcentrations may be used, and the ranges may overlap. It will beappreciated that the terms n and n+ (and similarly p and p+) are used todenote differences in the carrier concentration rather than absoluteconcentrations. The absolute concentrations may be tailored as desiredin order to achieve a certain performance characteristic. Examples ofpossible p-type dopants are boron, and possible n-type dopants includephosphorus, antimony and arsenic.

It has been explained above that there is an optimal distance betweenthe electrodes 112, 114 which balances the requirement that theelectrical signal in the electrodes not interfere with the photons inthe waveguide 122, with the requirement for the electrodes to be asclose to each other as possible. Effectively, it is the minimum distanceat which the former requirement is satisfied. The length is hereinafterdenoted L_(optimal), and it will be seen that the electrodes 112, 114 ofthe modulator 100 are separated by this distance. The electrodes 112,114 also have a thickness t.

FIG. 3 shows in plan view biasing circuitry of the modulator 100according to embodiments of the present invention. The biasing circuitrycomprises the electrodes 112, 114, as well as a further referenceelectrode 116 which is referred to as a secondary reference electrode.The signal electrode 112 is connected to a system which generates abiasing electrical signal. The primary reference electrode 114 isconnected to a reference signal, such as ground.

In general, the electrodes take the familiar coplanar arrangement. Theoptical part of the modulator runs between the signal electrode 112 andthe primary reference electrode 114, separated by L_(optimal) asdescribed above. The secondary reference electrode 116, however, isseparated from the signal electrode 112 by a gap L_(tuned), which isdifferent from L_(optimal). That is, the electrode arrangement isasymmetrical. In one embodiment, L_(tuned) is greater than L_(optimal).

As explained above, a wider signal electrode 112 provides a higherbandwidth in the electrical signal. However, increasing the width of thesignal electrode has a knock-on effect on the characteristic impedanceof the modulator (if the gap between signal and reference electrodes isnot also increased) or the efficiency of the modulator (if the gapbetween signal and reference electrodes is increased). According toembodiments of the present invention, the gap between the signalelectrode 112 and the primary reference electrode 114 (i.e. thoseelectrodes coupled to the waveguide 122) is kept at L_(optimal). Thisprovides optimal performance in the waveguide itself and the surroundingsemiconductor section. The gap between the signal electrode 112 and thesecondary reference electrode 116 is changed to a different valueL_(tuned), such that the characteristic impedance of the modulator 100is kept at the same value. This allows the signal electrode to have agreater width (and therefore a greater bandwidth) and yet keep the samecharacteristic impedance by appropriate adjustment of L_(tuned). In oneembodiment, the signal electrode 112 has a width in the range from 5 to20 μm.

In a further aspect of the present invention, the inventors have foundthat the bandwidth of the modulator 100 can be increased by increasingthe thickness t of the electrodes 112, 114. For example, FIG. 4 shows agraph illustrating the bandwidth of electrodes with differentthicknesses. The results were taken by an s-parameter measurement toolwhich measures the magnitude of the electrical signal at the end of theelectrode. The different lines represent different metal thicknesses.The original electrodes (dashed line) are 480 nm thick, and theparameter A represents the additional thickness of the electrodes overthe original. The solid line is therefore the result for a 900 nm thickelectrode and the dotted line for a 1350 nm thick electrode.

If the bandwidth of the electrode is lower than the frequency limitationof the semiconductor section (i.e. the waveguide 122) and any frequencylimitation placed upon the device 100 by an electrical signal-lightvelocity mismatch, then it will be the limiting factor in the high speedperformance of the device. With the original electrodes (of thickness480 nm) the device would be limited to 3 GHz operation even if thesemiconductor section is able to go much faster.

The skin depth is an important consideration when selecting anappropriate metal thickness for the electrodes. AC signals propagatealong a conductor close to the surface (or skin) and the skin depthdefines the distance into the conductor from the surface where thecurrent density has fallen by a factor 1/e. The skin depth δ isdependent on the frequency of operation f, the resistivity of theconductor ρ and the absolute permeability of the conductor μ accordingto the following expression:

$\delta = \sqrt{\frac{\rho}{f\; {\pi\mu}}}$

In order for the resistance of the AC signal (and therefore attenuation)to be as small as possible in one embodiment the electrode thickness isselected such that the AC current is spread through the conductor asmuch as possible. Therefore typically 2 to 5 skin depths should be usedat the frequency of operation. Aluminium is typically used in thebackend of standard CMOS processes and the skin depth of aluminium at 10GHz is approximately 850 nm. In one embodiment, therefore, an electrodethickness of 1.7 to 4.3 μm (or 1 to 5 μm) is used for operation at 10GHz. For combined front end photonic-electronic integration schemeswhere photonic and electronic components are fabricated side by side onthe same substrate and therefore share the same backend, this can beproblematic since the first metal layer can be as thin as 500 nm. Inthis case, where there is no flexibility in the metal thickness, it isdesirable to form the electrodes on higher metal layers which havegreater thickness and to contact down to the semiconductor layer.

FIG. 5 shows an optical communication system 150 according toembodiments of the present invention. A driving system 152 provides abiasing electrical signal V_(bias) to the signal electrode 112;reference electrodes 114, 116 are connected to a reference signal input,e.g. ground. The driving system 152 has an impedance of typically 50Ohms. The value of L_(tuned) is therefore chosen such that thecharacteristic impedance of the modulator 100 is equal to that of thedriving system 152.

An optical input to the waveguide 122 is provided by a source of photons154. The modulated optical signal is output to further components of thesystem, illustrated generally by the reference numeral 156.

It was shown by the inventors of the present application thatimplementation of silicon optical modulators in optical communicationssystem allows achieving transmission rates up to 50 Gb/s, see, forexample “High contrast 40 Bgit/s optical modulation in silicon” by D. J.Thomson et al., Optics Express, v. 19, No. 12, p. 11507 (2011) and“50-Gb/s silicon optical modulator” by D. J. Thomson et al., IEEEPhotonics Technology Letters, v. 24, No. 4, p. 234 (2012).

The present invention thus provides an electro-optic modulator and anoptical communication system in which a wider signal electrode may beused without affecting the characteristic impedance of the device or theefficiency of the optical modulation. In embodiments of the invention,asymmetric coplanar electrodes are provided such that the gap betweenthe signal electrode and one reference electrode may be optimized forthe optical waveguide and the semiconductor section surrounding it, andthe gap between the signal electrode and the other reference electrodemay be optimized for a particular characteristic impedance.

It will of course be understood that many variations may be made to theabove-described embodiment without departing from the scope of thepresent invention. In particular it will be understood that the presentinvention is not limited to the illustrated arrangements of dopedregions. For example, the reverse arrangements are also possible, inwhich p-type and p+ type regions are replaced with n-type and n+ typeregions and vice versa. Various positions and shapes of the junctionbetween the n-type and p-type regions are also possible. Furthermore, itwill be understood by those skilled in the art that the modulator designset out in FIG. 2 is illustrative of one possible design which may beused with the electrode design described above and with respect to FIG.3 in particular. The electrodes could also be applied to othermodulators based upon other effects, such as carrier accumulation andinjection, as well as modulators formed in other photonic materials.

1. An electro-optic modulator comprising: an optical waveguideintegrated in a layer of silicon; and a biasing circuitry for applyingan electric potential across the waveguide, said biasing circuitrycomprising: a signal electrode coupled to one side of the waveguide, forapplication of an electrical signal; a primary reference electrodecoupled to the other side of the waveguide, for application of areference signal; and a secondary reference electrode, for applicationof said reference signal, located such that the signal electrode lies ina coplanar arrangement between the primary reference electrode and thesecondary reference electrode, wherein a first width of a first gapbetween the signal electrode and the primary reference electrode and asecond width of a second gap between the signal electrode and thesecondary reference electrode are not equal.
 2. The electro-opticmodulator according to claim 1, wherein the first width is such that thesignal electrode and/or the primary reference electrode does notinteract with a light propagating in the waveguide.
 3. The electro-opticmodulator according to claim 2, wherein the second width is such that acharacteristic impedance of the biasing circuitry takes a desired valueto achieve an improved modulator performance compared to a performanceof a modulator with equal the first and the second gap widths.
 4. Theelectro-optic modulator according to claim 1, wherein the second widthis such that a characteristic impedance of the biasing circuitry takes adesired value to achieve an improved modulator performance.
 5. Theelectro-optic modulator according to claim 4, wherein the improvedperformance is an increased modulation bandwidth.
 6. The electro-opticmodulator according to claim 1, wherein the signal electrode is elongateand runs parallel to the primary and secondary reference electrodes. 7.The electro-optic modulator according to claim 1, wherein the signalelectrode has a width in the range from 5 to 20 μm.
 8. The electro-opticmodulator according to claim 1, wherein at least the signal electrodeand the primary reference electrode have a thickness in the range from 1to 5 μm.
 9. The electro-optic modulator according to claim 1, whereinthicknesses of the electrodes may be different and is optimized forimproved performance.
 10. The electro-optic modulator according to claim1, wherein the optical waveguide is formed in a first layer togetherwith electronic components, and wherein the biasing circuitry is locatedon a second, higher layer with greater thickness than said first layer.11. The electro-optic modulator according to claim 1 which is arrangedsuch that modulation is effected by the plasma dispersion effect.
 12. Anoptical communication system, comprising: a source of photons, coupledto an optical waveguide of an electro-optic modulator, the modulatorcomprising the optical waveguide integrated in a layer of silicon; and abiasing circuitry for applying an electric potential across thewaveguide, the biasing circuitry is driven by electrical signalsprovided by a driving circuitry; and said biasing circuitry comprising:a signal electrode coupled to one side of the waveguide, for applicationof an electrical signal; a primary reference electrode coupled to theother side of the waveguide, for application of a reference signal; anda secondary reference electrode, for application of said referencesignal, located such that the signal electrode lies in a coplanararrangement between the primary reference electrode and the secondaryreference electrode, wherein a first width of a first gap between thesignal electrode and the primary reference electrode and a second widthof a second gap between the signal electrode and the secondary referenceelectrode are not equal.
 13. The optical communication system accordingto claim 12, wherein the second gap has a width such that acharacteristic impedance of the biasing circuitry is equal to acharacteristic impedance of the driving circuitry at a boundary betweenthe two.
 14. The optical communication system according to claim 12,wherein the electrical signal alternates at a frequency f, resulting ina skin depth δ, and wherein at least the signal electrode and theprimary reference electrode have a thickness in the range from 2δ to 5δ.15. The optical communication system according to claim 12, wherein theoptical waveguide is formed in a first layer together with electroniccomponents, and wherein the biasing circuitry is located on a second,higher layer with greater thickness than said first layer.
 16. Anoptical modulator for embedding data on an optical beam, comprising: anoptical waveguide integrated in a layer of silicon, and an electrodesystem with at least three parallel electrodes applying an electricpotential across the waveguide; the electrode system being asymmetricwith a first gap between a first and a central electrode and a secondgap between a second and the central electrode, wherein widths of thefirst and the second gaps are different; wherein the first width isoptimized for the optical waveguide to embed data; and the second widthis optimized to set the characteristic impedance of the modulator toachieve the improved performance.
 17. The optical modulator of the claim16, wherein the improved performance is in better modulationcharacteristics compared with a modulator having equal the first and thesecond gap widths.
 18. The optical modulator of the claim 16, wherein athickness of the electrodes is selected to allow an AC current to spreadthrough a conductor to improve the modulator performance.
 19. Theoptical modulator of the claim 16, wherein the modulator operates in along haul optical communication system with high data rates up to 50Gb/s.
 20. The optical modulator of the claim 15, wherein the modulatoroperates in a radio-over-fiber links.